The proposed investigations are designed to test prospectively the clinical and immunologic responses of 6 mo infants to measles vaccine. Two measles vaccines, AIK-C and Connaught, with differing strain derivation would be tested in standard titers. Infants whose maternal immunization history is known would be recruited because initial findings based on detailed, reliable archival immunization records show that the level of passive maternal measles antibody and seroresponse of 6 mo infants correlates with the maternal immunization history. Both serologic and cellular (CMI) responses to vaccination would be evaluated. CMI measurements are of particular significance in light of evidence of immunosuppressive effects associated with high titered measles vaccines, affecting especially girls. Data from this trial would supply immunologic data essential in measles vaccination decisions of the future. Trial infants would receive measles vaccine at 6 mo (at the same time as 3rd DPT and Hib) and MMR at 15 mo. Four blood samples would be taken: pre-6 mo-vaccination at 6 mo, post-6 mo-vaccination, post-MMR and 12 mo after MMR. Serologic measurements would be done by hemagglutination (HA) inhibition, enzyme immunosorbent assay, plaque reduction neutralization and neutralization tests. CMI tests would be done on peripheral blood lymphocytes (PBL) by blast transformation in presence of measles HA antigen, and by release by PBL in 3 day cultures of the cytokines interleukin-2 (11-2) and tumour necrosis factor (measures of T-helper (TH) type 1 activity), Il-4 and Il-5 (measures of TH2 activity), and enumeration of CD4 and CD8 cells on each sample. 220 infants would be recruited specifically to evaluate seroconversion (Part A) and 160 for serology and CMI (Part B). For Part A, the "post-" blood samples would be taken 8 weeks after each vaccination and for Part B, at 4 weeks. All samples (Part A and B) would be tested serologically. Each post,-vaccination sample from Part B would be evaluated for CMI. A control group of infants would be recruited who have received the third DPT-Hib vaccine at 6 mo, but measles vaccine only as MMR at 12 mo. Blood samples for CMI and serology would be obtained from these 4 weeks after the 6 mo (as age matched controls) and MMR vaccinations, and for serology 12 mo after MMR. Test results would be compared between infants grouped by vaccine received, maternal immunization status, and gender, and between serologic and CMI results.